(bug 15466) Added action=purge to the API. Modified patch by ^demon

Files:
modified: /trunk/phase3/includes/AutoLoader.php (try)
added: /trunk/phase3/includes/api/ApiPurge.php (try)
modified: /trunk/phase3/includes/api/ApiMain.php (try)
modified: /trunk/phase3/RELEASE-NOTES (try)
modified: /trunk/phase3/includes/api/ApiBase.php (try)